Method of making embossed battery separators



Sept. 13, 1955 E. C. UHLIG ET AL METHOD OF MAKING EMBOSSED BATTERYSEPARATORS 3 Sheets-Sheet 1 Filed Jan. 4, 1952 INVENTORS w/z/a x N HNEWATTORNEY Sept. 13, 1955 E. c. UHLlG ET AL 2,717,423

METHOD OF MAKING EMBOSSED BATTERY SEPARATORS Filed Jan. 4, 1952 3Sheets-Sheet 2 mexm ea A1 TORNEY' Sept. 13, 1955 E. c. UHl.-IG ET AL2,717,423

METHOD OF MAKING EMBOSSED BATTERY SEPARATORS Filed Jan. 4, 1952 :sSheets-Sheet 5 BY 3 5 21 {M 4. mm

A .TTORNEY United States Patent METHOD OF MAKING EMBOSSED BATTERY ISEPARATORS Edwin C. Uhlig, Greenwood, and Linwood A. Murray, Jr.,Cranston, R. I., assignors to United States Rubber Company, New York, N.Y., a corporation bf New Jersey This invention relates to a method ofmaking embossed battery separators of cellulose fibrous sheet materialwhich has been impregnated with a thermosetting resin, and of heatcuring the embossed sheet to set the impregnating resin.

Most of the electric storage batteries manufactured today for theautomobile industry are provided with wood separators, cut from wood soas to provide ribs upon one face thereof, and used to separate the leadplates in the battery. Wood separators are used because they are lessexpensive than the other suitable materials, such as microporous hardrubber separators, matted glass fiber separators etc., but woodseparators have a processing disadvantage in that the wood must be wettreated and the separators must be handled and maintained in the wetcondition until submerged in the electrolyte of Wood separators haveanother disadvantage in that they cannot be used in the dry-charged typeof battery,

since they must be maintained in the wet condition.

The above mentioned microporous hard rubber separators are suitable fordry-charged storage batteries and have many advantages over the woodseparators, but they are too expensive to compete for the marketserviced by the wood separators.

The present invention contemplates an inexpensive battery separatorformed from a bibulous cellulose fibrous sheet such as wood pulp orcotton linter and which is impregnated with a carefully selectedthermosetting resin, which when cured, will protect the cellulose fibersfrom the action of the battery electrolyte and will also increase thestrength of the sheet, help to hold the fibers in place and impart tothe separator sufficient stiffness to cause it to retain the desired ribformation after the sheet has been embossed and cured.

The battery separator contemplated by the present invention can bemanufactured at a sufficiently low cost to compete in price with theabove mentioned wood battery separator, and it possesses an importantadvantage f over the wood separator in that it does not need to be keptwet prior to its introduction into a battery and immersed in theelectrolyte of the battery.

The manufacture of an embossed, resin treated battery separator formedof a sheet of cellulose fibrous material, as herein contemplated,presents a number of problems. One of these problems is due to the factthat a fibrous sheet such as a sheet formed of wood pulp or cottonlinter tends to distintegrate when soaked in battery acid, and thereforeit must be impregnated with a sufiicient amount of resin to strengthenthe sheet and protect the fibers from the battery acid. On the otherhand a battery separator must be sufficiently porous to permit theelectrolyte to pass fairly freely therethrough- Therefore, anotherrequirement of the resin used to impregnate the sheet to strengthen andprotect the same 2 the method of the present invention are to providean.

Patented Sept. 13, 1955 ICC Still another problem encountered indeveloping the method of the present invention arose in embossing the.

fibrous sheet to provide the same with the desiredembossed ribs. Inorder to produce such an embossed separator it is important that theembossing operation be performed at high speed so as to keep the costdown, but it is found that if the fibrous sheet. is rapidlyembo'ssed'itwill tend to crack along the embossed or distendedribs. These cracks arecaused when the essentially non-elastic fibrous sheet is quicklystretched as it is passed between rapidly turning embossing rolls. Wehave found that one way to prevent such cracking of the fibrous sheetwhen it is being embossed at high speed is to'cover each face with aliner of sheet material so that the pressure of the embossingrolls willbe exerted upon the liner sheets, which engage and protect the fibroussheet as it is embossed. These liner sheets act to distribute the strainexerted upon the fibrous sheet and to mechanically support such sheetduring the embossing operation. It is found that the use of suchlinersheets make it possible to emboss the fibrous sheet at speed withoutproducing cracks along the embossed n s.

Another method for relieving the fibrous sheet from serious strainsduring the embossing operation is topress the advancing fibrous sheet atsuccessively advanced points into the grooves of an embossing roll, tothereby initiate the formation of embossed ribs, and then complete theembossing operation by passing the sheet between embossing rolls. ,Thefibrous sheet is preferably. embossed while it is in a slightly damp andlimpcondition.

As soon as the fibrous sheet has been embossed and while it retains mostof the embossed design it should be heat treated to cure thethermosetting resin with which the sheet was impregnated beforetheembossing operation, so as to set the resin and increase the stiffnessof the embossed sheet. I

' Having in mind the foregoing, the primary objects of inexpensiveembossed battery separator formed of a bibulous cellular fibrous sheetthat is so impregnated 1 with a thermosetting resin that the resinrenders the separators sufficiently strong and durable to give goodservice-in the battery, while such resin does not render the separatorimpervious to the electrolyte of the bat-f tery; and to produce suchseparator at a sufiiciently low cost to enable it to be sold incompetition with the wood battery separators which are now in extensiveuse in the batteries used by the automobile industry.

The above and other features of the method of. the present inventionwill be further understood from the following description when read inconnection with the accompanying drawings showingapparatus for carryingout the method.

In the drawings:

Fig. l is a more orless schematic view of apparatus for embossing, heatcuring, and cutting 'an advancingsheet into separators, the view beingin side elevation with parts in section;

- Fig. 2 'on a larger scale is a vertical sectional view throughapparatus for impregnating, and drying the fiber sheet preparatory toembossing the same;

of Fig. l; H

Fig. 5 is a vertical sectional view through a modified the view beingtaken on the line form of embossing apparatus for embossing theadvancing sheet;

Fig. 6 is a top plan view of the apparatus shown in Fig.

Fig. 7 is a top plan view of a fibrous sheet showing how the sheet isprogressively initially embossed by the apparatus of Fig. 5 before it isacted upon by the embossing rolls;

Fig. 8 is an enlarged sectional view taken on the line 8-8of Fig. 5; iFig. 9 is a similar view taken on the line 9-9 of Fig. 5;

Fig. 10 is a transverse sectional view through a finished embossedseparator such as can be produced by the ap; paratus shown on Sheet 1 ofthe drawing, or by nemesified' apparatus shown in Figs. Sand 6 of thedrawing; and

I Fig. 11 is a face view of the embossed separator of Fig. 10.

The nature of the fibrous sheet used in carrying out the presentinvention is important if good battery separators are to be produced.What is required is a 'bibulous cellulose fibrous sheet formed of cottonlinter, or wood pulp which has been freed of most of the natural resinsto provide a fibrous sheet containing at least 85- percent alphacellulose. This fibrous sheet should be uniform as to thickness and asto 'fiber di'stributiomythat is it should have what is called uniformformation. The sheet may have a gauge of about .03" and should be porousso that the resin with which the sheet is to be impregnated canpenetrate deep into the sheet to protect and unite the fibers. i

The type of binding agent used to impregnate. the fibrous sheet is alsoimportant. The preferred'binding agent is a thermosettingphenol-formaldehyde resin that is preferably employed in the form of asolution of the A-stageresin in an aqueous medium such as water oraqueous acetone or aqueous lower aliphatic alcohol, e. g.', methanol,ethanol or isopropanol. However, other thermose'tting resins which areacid-resistant such as phenolfurfural resins, melamine-formaldehyderesins, etc.,"may be used in the form of a solution or suspension and asabove stated the resin should be so introduced into the fibrous sheetthat it will stiffen the sheetwhen cured and protect the fibers, withoutunduly decreasing the porosity, of the fibrous sheet. This resinsolution should b'suffi ciently thin to penetrate the entire sheet andbecome deposited around'the fibers.

Now referring to Sheet 1 of the drawings, the bibulous cellulose fibroussheet which is to be impregnated with a resin, such as above described,is. preferably provided in the form of the roll of porous paper stocksuch as designated by P in Fig. 2 of the drawing, and the width of thislong strip of paper is preferably slightly greater than that of thefinished separator, so that a small amount of stock can be trimmed fromeach edge of the embossed sheet to produce a finished separator of the.desired width. The supply roll 10 of fibrous sheet material R ispreferably wound upon a supporting core, not shown, torotate upon ashaft 11. The core hassecured thereto a brake wheel 12 so that theunwinding of the paper sheet P from the package 10 will be retarded by abrake band 13 engaging this wheel, and one end of this band is anchoredand the other end supports the weights 14. This sheet P is shown aspassing from the supply package 10 downwardly into the tank 15 thatcontains the above described impregnating resin X mixed with a largeamount of water. The sheet P passes around the rollers 16 disposed belowthe level of the liquid X and as it leaves the tank 15 it passesupwardly between the squeeze rolls 17 which squeeze from the sheet theexcess liquid. This sheet then passes through an opening 18 in the dryerhousing 19 and it travels in this dryer in the directionindicated by thearrowsabout the guide rolls 20, and while. the sheet passesv betweensuch rolls it is subjected to the bossed ribs.

. 4 drying action of the heating lamps 21. The fumes from the dryingaction within the dryer are carried off through the discharge pipe 22.The impregnated sheet P is pref erably dried within the dryer 1sufficiently to remove most but not all of its moisture and upon leavingthis dryer it preferably retains from about 10 to 15% moisture, basedupon the weight of the dry fibrous sheet plus resin. As this sheetleaves the dryer it passesthrough an opening 23 and is then wound uponthe roll 24, which roll is driven by the gear 25. This 10 to 15%moisture causes a desired swelling of the paper sheet and renders itmore flexible.

The impregnated sheet wound into the roll 24 maybe transferredimmediately to the apparatus shown in Fig. l and mounted in the positionindicated by 24' at the left hand end of such apparatus. If however asubstantial length of time, such as one or a number ofdays is to elapsebefore the impregnated sheet of the roll 24 is tobe embossed, then thisroll should be placed in a moisture tight envelope and in cold storageto prevent the'sheet from drying out, and to prevent the resin thereinfrom setting before the sheet is embossed.

Now referring more particularly to Fig. 1, it will -be seen that theresin treated supply roll 24 is mounted at one end of the apparatusshown and is provided with the brake retarding means26 which serves toplace some tension upon the impregnated sheet, now designated by I, asit unwinds from the supply package 24. This sheet I passes from thesupply roll over a guide rod 27 and then in 'a horizontal direction in astraight'line throughout practically the entire machine. 7

It is found desirable to heat the resin impregnated sheet I to atemperature of approximately 200 F. before it is subjected to theembossing operation, as this heat together with the small amount ofmoisture present in the sheet serves to increase the softness andflexibility of the sheet so that the embossing operationwill be lesslikely to crack the'paper sheet.

The embossing'operation is preferably performed 'by advancing the sheetI ate uniform speed between embossing rolls, and it is found that if theembossing rolls are rotated at a 'slowspeed the hot moist sheet I can beembossed without causing cracks to form along the ern- But hisimportant, in order to reduce manufacturing costs, to carry out theembossing operation-at high speed, and his found that if the sheet I isadvanced.

rapidly between the embossing rolls cracks are likely tobeproduced'along the embossed ribs, unless steps such as hereinafterdescribed ar e taken to prevent the formation of such cracks.

The sheet I is shown as passing from the roll 27 horizontally betweenthe electrically heated plates 28 and 29 that are supported in slightlyspaced relation to each other by the machine frame 3%. These pre-heatingplates serve to heat the advancihg sheet I to a temperature of about 200F. before it reaches the embossing rolls 31 and 32 which are rotatablysupported by the frame 33, and these rolls or. one of the is driven bythe motor 34. These rolls are not heated but some heat will be suppliedto them by rapidly to a severe stretching operation by the distorting;

action of the male and female'roll, it may crack at the ribs. We havefound that such cracking can be prevented by covering each face of thefibrous sheet with a protecting apron or liner during the embossingoperation. These aprons reinforce the paper sheet during the embossingoperation and apparently permit some fibrous slippage in the sheetacross the embossing surfaces to thereby lessen the strain onthatportion of the fibrous sheet which being displacedduring-theernbos'sing operation.

One means for covering the fibrous sheetwith protcct-' ing aprons duringthe embossing operation is shown in Fig. 1 of the drawing wherein theupper protecting apron or liner is indicated by and the lower protectingapron is indicated by 36. These liners may be formed of metal foil or ofa thin plastic film but are preferably formed of a laterally stretchablefabric. Such liners preferably are given the form of endless belts thatpass between the embossing rolls with the traveling sheet I, and eachliner or belt is made relatively long as shown to reduce the wear uponthe same resulting from their repeated passage between the rolls 31 and32, as these embossing rolls tend to wear the liners out rapidly. It isfound that the life of such liners can be increased substantially byforming each apron of a thin woven stretchable fabric which is renderedsomewhat elastic transversely by coating the surface of the same thatcontacts the sheet I with rubber to form a somewhat elasticrubber-backed stretchable fabric. The life of such liners may be furtherincreased by applying tale to the face thereof that engages theresin-treated sheet I, to

thereby prevent such resin as may be picked up from the sheet I fromrendering the face of each liner tacky. When fabric liners are used theyare compressed by the embossing pressure and this produces a distributedpressure on the sheet I at the embossing points, which is desired.

In the construction of Fig. l the upper endless liner 35 is shown aspassing arouhnd the guide rolls 37, and the desired tension ismaintained upon this liner by mounting one of these rolls 37 for slidingmovement as indicated by 38 so that this roll is continuously urged inthe belt tight: ening direction by a weight 39. The lower apron 36passes around the guide rolls 40 and one of these rolls 40 is slidablymounted as indicated at 41 and is urged in the belt-tightening directionby the weight 42. Talc is applied to one face of the upper belt 35 by arotating brush 4?: which rotates in the talc box 44 and is driven by thechain 45. Talc is similarly applied to one face of the lower belt 36 bya brush 46 which rotates in the tale box 47, and this brush is driven bya chain 48 from the motor 49. The manner in which the aprons 35 and 36protect or strengthen the sheet 1 during the embossing operation isshown in Fig. 3.

The action of the crimping rolls 31 and 32 is preferably such that theyover-crimp the sheet I slightly, and then the height of the ribs Rformed by the crimping operation is reduced to the desired dimension bypassing the freshly crimped sheet between the accurately spaced rolls50. It is found that this procedure of slightly over-crimping the sheetand then passin it between the accurately spaced rolls 50 gives goodcontrol of the height of the embossed ribs R.

The embossed sheet passes from the rolls 50 through a relatively longcuring chamber where it is subjected to the heat of the electric heaters51 and 52. The upper heater is preferably enclosed in the housing 53 andthe lower heater in a housing 54, as best shown in Fig. 4. The upperhousing is preferably pivotally secured to the lower housing by thehinges 55, so that the upper housing can be raised by the handle 56 whenaccess to the interior of the curing oven is desired. These enclosedelectric heaters preferably subject the advancing embossed sheet I to Ha temperature of 500 F. to 550 F., for a sufiicient length of time tocure the resin with which the sheet is impregnated. This curingapparatus shown in Fig. l is supported by the uprights 57.

As the embossed and cured sheet I leaves the curing chamber it ispreferably sprayed with a wetting agent as indicated at 58 and 59 as theintroduction of this wetting agent into the pores of the sheet causes itto absorb the electrolyte quickly when the separator is first broughtinto contact with the same. The embossed separator then preferably hasits sides trimmed by the rotating cutter discs 60 driven by the belt 61,to give this sheet I the width desired in the finished separator. Theembossed sheet I is shown as drawn through the curing oven and past therotating cutters 60 by the rolls 62 driven by a chain 63 from the motor64. This completes the formation of the battery separators except forcutting the advancing sheet I transversely into the battery separatorsS. To effect this cutting operation the sheet I is advanced to a cutterby the reciprocating means 65, and the cutter comprising the fixedcutter block 66 and reciprocating cutter 67 cut the sheet I into theseparators S. The cut separators are advanced by an endless belt 68 thatis supported and driven by the rollers shown and the separators form astack not shown. The finished separator S may have the appearance bestshown in Figs. 10 and 11, in which itwill be seen that the embossedseparator has the raised hollow ribs R extending longitudinally thereof,and formed by dis: placing laterally the entire thickness of therib-forming stock.

By employing the construction above described and using the aprons 35and 36 to protect the fibrous sheet during the embossing operation it isfound practical to carry out the embossing operation at high speed, sayat the rate of 60 feet per minute without causing cracks to form alongthe ribs, but due to the wear upon the protecting aprons 35 and 36caused by the severe pressure to which they are subjected by theembossing rolls, it is necessary to replace these aprons from time totime.

It is found that highly satisfactory embossing results can be secured ifthe protecting aprons just mentioned are omitted, provided that aninitial crimping or embossing operation is performed upon the fibroussheet I before it is subjected to the embossing action of the embossingrolls. This initial embossing operation may be imparted to the sheet Iby employing the modified construction shown in Figs. 5 and 6 of thedrawing and which will now be described. The sheet I shown in Figs. 5and 6 is preferably preheated similar to the manner above-described, bypassing it between the heaters 28' and 29 so that when it leaves thispre-heater it passes under a guide roller 69 and then around an upper orfemale embossing roll 31'. It then passes between the nip of this upperroll 31' and a slightly ribbed lower roll 32. These rolls are rotatablysupported by the frame 33 and are driven by means, not shown. Theadvancing sheet I is pre-crimped while it engages the female roll 31'.This is accomplished by providing means for forcing the sheet Iprogressively into the grooves of the female roll 31. To this end, a

- series of endless wires may be employed to force the sheet stock intothe grooves of the roll 31', and these endless wires are arranged, asshown, to progressively press the sheet stock I into the recesses of theroll 31' in the order shown in the plan view of Fig. 7, where it will.be seen that the central rib r is formed first by drawing the sheetmaterial inwardly in a lateral direction from its sides. Then the nextpair of ribs r' are initially formed, and then the pair of ribs r" areinitially formed. In this way the stock is drawn inwardly from thelateral sides as needed to form the embossed ribs, so that each rib isproduced without appreciably stretching the sheet forming the same.These endless wires 70 pass around the guide roller 71 and are tensionedby adjusting the tension roll 72. The first wire 70 which forms thegroove r passes around a guide rod 73. The wires which form the groovesr' pass around the rod 74, and the wires that form the grooves r" passaround the rod 75, and so on until each groove or rib shown in Fig. 7 isproduced in the sheet. It willbe noted that all of the wires 70 curvearound and are driven by the roll 31 and pass between the nip of therolls 31 and 32' ployed to produce at high speed the separator shown inFigs. 10 and 11 of the drawing or if desired the modified constructionshown in Sheet 2 of the drawings may be Battery separate-rs formed inaeeordance with the 't rite {1d are relatively iheitp'ensive to produce,as the e lu ose fibrous sheet used is not 'eirpensive and the amol'iii t"of resin used to stiifen the sheet and protect the l is small. Theseseparators have low electrical resi nee and are round in practice togive satisfactory and long seryloe lrl batteries of the type fised inautomobiles.

Having thus described our invention, what we claim and desire to protectby Letters Patent is:

1. The method or making porous embossed battery separators through whichan ele'otrolyte can pass which comprises, providing a bibulous cellulosefibrous sheet, impre'gha ting this sheet as to deposit the same in andaround the fibers, drying the if pregnated fs heet sunset-easy to removemost of its moisture and while it cori'tains a small amount of moistureembossirigthe sheet so as to form laterally-spaced raised hollowjr'ibsby advancing it between rotating embos'sin'g rolls thatsomewhat oiieremboss the sheet, then passing the sheet between accurately spacedsmooth rolls that "pres'sthe ribs dowh to a predetermined height, thenheating thiserjnbossed sheet to cure the resin, and cutting the sheetinto ba ttery separators.

2. The method of making porous embossed battery separators through whichan "electrolyte can pass which eompr'iscs, providing a bibulouscellulose fibrous sheet, impregnating this sheet with an aqueoussolution of an A -jstag'e phenol-formaldehyde resin so as to deposit theresin in and around the fibers, drying the impregnated sheet to avolatile content of from 10 to 15% by weight based upon the weight ofthe basic fiber sheet plus resin, then oovering 'each face 'ofsuch sheetwith a protecting of laterally stretchable sheet material that issufficicntlys'trctchable so that the sheet and protecting aprons will"stretch laterally together in the area of each rib, embossing the sheetwith laterally spaced hollow ribs by advancing the apron-sheet sandwichrapidly and continuously between rotating 'embossin'g rolls to cmbossthe sheet \fyith ribs spaced laterally of the sheet and extending alongthe length of the sheet, removing the aprons, then immediately passingthe'embos'sed sheet through a highly heated passage continuously to heatthe embossed sheet to cure the resins so that th'e sheet remains porousto an electrolyte, and cutting the sheet into battery separators. 3. Themethod of making porous embossed battery separators through which anelectrolyte can pass which comprises, providing a'bibulous cellulosefibrous sheet of tlen'gth, advaneifig this sheet through a bath ofthermosetting "resinous bonding agent dissolved in an atluedtis niedmmcontinuously to impregnate the sheet so as to deposit the resin "in andaround the fibers, drying the impregnated sheet sufiiciently to removemost of its moistiire, andwhile it contains a small amount of moistirreembossingihe sheet with laterally spaced hollow ribs by ad'yaiicl 1tcontinuously. and rapidly between rotatingensessmgro11s to emboss ribswhich are spaced laterally of the sheet and exten along the length ofthe slie eh'aiid coveringeach face of the advancing sheet while it isbeing embossed with a laterally stretchable apron com-posed era materialthat is sufficiently stretchable so that the "sheet and protectingaprons will stretch laterally together in the area of "each rib, thenadvancing immediately the sheet rapidly and continuously between spacedheating platens to cure 't heshee't 'so that it remains pe rvious to"an"elect rolyte, and cutting the sheet info battery parator's. I g

V e method or making porous embossed battery separators throiig h whichan "electrolyte can pass which ith a resinous bonding agent so 8comprises, providing a 'bib ulo us cellulose fibrous sheet,irripreghatirig this sheet with a resinous bonding agent so as todeposit the same in and around the fibers, drying t'h impreghated sheetsufiiciently to remove most of its moisture 'ahd while it contains asmall amount of 'moistore embossing the sheet so as to form laterallyspaced raised hollow ribs by advancing it between rotating embossingrolls and providing at each face of the sheet a laterally stretchableprotecting apron composed of a material that is suflieie'ntlystretchable so that the sheet and protecting apron's will stretchlaterally together in the area of each rib, then immediately heatingthis embossed sheet to cure the resin, and cutting the sheet intosatiety separators.

5. The method or making porous embossed battery separators through whichan electrolyte can pass which comprises, providing a bibulous cellulosefibrous sheet, impregnating this sheet with a resinous bonding agent soas to deposit the same in and around the fibers, drying the impregnatedsheet to a volatile content of from 10 to 15% by weight based upon theweight of the basic fiber sheet plus resin, embossing the sheet so as toform laterally spaced raised hollow ribs by advancing it betweenrotating embossing rolls and providing at each face of the "sheet alaterally stretchable protecting apron composed of 'a material that issufiiciently stretchable so that the 'sheet and protecting aprons willstretch laterally together in the area of each rib, then immediatelyheating this embossed sheet to cure the resin, and cutting the sheetinto battery separators.

'6. The method of making porous embossed battery separators throughwhich an electrolyte can pass which comprises, providing a bibu'louscellulose fibrous sheet, impregnating this sheet with a resinous bondingagent so as 'to deposit the same in and around the fibers, drying theimpregnated sheet 's'umciently to remove most of its moisture and whileit contains asmall amount of moisture embossingthe sheet with laterallyspaced hollow ribs by advancing it rapidly between rotating embossingrolls and providing at each face of the sheet a laterally stretchableprotecting apron composed of a material that is sufiiciently stretchable'so that the sheet and protecting apron will stretch laterally togetherin the area of each rib, then advancing the embossed sheet through ahighly heated passage to cure the resin so that the sheet remainspervious to an electrolyte, and cutting the sheet into batteryseparators.

7. The method of making porous embossed battery separators through whichan electrolyte can pass which comprises, providing a bibulous cellulosefibrous sheet,

impregnating this sheet with an aqueous solution of an A-stag'e phenolformaldehyde resin so as to deposit the resin in and around the fibers,drying the impregnated sheet sufficiently to remove 'most of itsmoisture and while it contains a small amount of moisture embossing thesheet so as to form laterally spaced raised hollow ribs by advancing itbetween rotating embossing rolls and providingat each face of the sheeta laterally stretchable protecting apron composed of a material that issufficiently stretchable so that the sheet and protecting aprons willstretch laterally together in the area of each rib, removingthe'ap'ronsand then heating the embossed sheet to cure the resin so that the sheetremains porous to an electrolyte, and cutting the sheet into batteryseparaors.

8. The method of making porous embossed battery separators through whichan electrolyte can pass which comprises, providing a bibulous cellulosefibrous sheet, impregnating this sheet with a thermostetting resinousbonding agent dissolved in an aqueous medium so as to deposit the resinin and around the fibers, drying the impregnated sheet sufiiciently toremove-most'oi its moisture and while it contains a small amount ofmoisture embossing the sheet with laterally spacedhollow ribs byadvancing it rapidly between rotating embossing -rolls-and Iproviding ateach face of the sheet while it is being embossed 9 a laterallystretchable protecting apron having talc on the sheet engaging face andcomposed of a material that is sufliciently stretchable so that thesheet and protecting apron will stretch laterally together in the areaof each rib, then curing the sheet so that it remains pervious to anelectrolyte, and cutting the sheet into battery separators.

References Cited in the file of this patent UNITED STATES PATENTS785,786 Wolfi Mar. 28, 1905 942,852 Backeland Dec. 7, 1909 1,206,983Bliss Dec. 5, 1916 10 Adams June 1, 1926 Russell Dec. 16, 1930 IewettFeb. 11, 1936 Sutherland Dec. 17, 1940 Smith Jan. 14, 1947 Wilson et a1.Apr. 8, 1952 Merrill Jan. 27, 1953 OTHER REFERENCES

2. THE METHOD OF MAKING POROUS EMBOSSED BATTERY SEPARATORS THROUGH WHICHAN ELECTROLYTE CAN PASS WHICH COMPRISES, PROVIDING A BIBULOUS CELLULOSEFIBROUS SHEET, IMPREGNATING THIS SHEET WITH AN AQUEOUS SOLUTION OF ANA-STAGE PHENOL-FORMALDEHYDE RESIN SO AS TO DEPOSIT THE RESIN IN ANDAROUND THE FIBERS, DRYING THE IMPREGNATED SHEET TO A VOLATILE CONTENT OFFROM 10 TO 15% BY WEIGHT BASED UPON THE WEIGHT OF THE BASIC FIBER SHEETPLUS RESIN, THEN COVERING EACH FACE OF SUCH SHEET WITH A PROTECTINGAPRON OF LATERALLY STRETCHABLE SHEET MATERIAL THAT IS SUFFICIENTLYSTRETCHABLE SO THAT THE SHEET AND PROTECTING APRONS